Various techniques are known for connecting a component such as a microchip (chip) to electrical circuitry on a printed circuit board (substrate).
Wire bonding is the oldest traditional approach. In this technique, the component is accurately placed in position (to within +/−30 microns) on the substrate, with contacts on the upper surface of the component (remote from the substrate) adjacent to respective associated contacts on the upper surface of the substrate. The contacts on the component are in the form of leads, 200 microns in length extending upwardly from the component. The component is secured in position on the substrate by die bonding with an adhesive. A wire bonding machine places a length of wire, typically 25 microns diameter gold or aluminium wire, to extend between a pair of associated contacts (one on the component and one on the substrate) and bonds the ends of the wire to the contacts, e.g. by ultrasonic or thermal welding. The wire bonds at this stage are fragile, exposed loops of metal that protrude upwardly from the substrate surface, and usually the wires are not insulated. The wire bonds are therefore typically encapsulated, e.g. in silicone material, to protect them from damage and insulate them, preventing contact between wire bonds and possible short circuiting.
Because the wire bonds protrude from the surface, this method is not ideal for flat products such as smart cards or RFIDs (radio frequency identification devices). Although this process can be fully automated, it is expensive and can cost in the region of 10 cents (US) per chip for a processor with many connections. The output rate is slower than desired, with a typical automated machine outputting around 10,000 components per hour. Also, the output rate is linked to the complexity of the component or number of bonds to be made; therefore more complex components are produced more slowly.
Another approach is tab bonding. In this technique, the substrate is provided with a hole slightly larger than the footprint of the component, with contacts appropriately located on the upper surface of the substrate adjacent the hole. The component has contacts in the form of outwardly extending, horizontally oriented wires or legs. The component is placed part way into the hole, with the protruding legs in contact with the associated contacts on the substrate. The pairs of associated contacts (one on the component and one on the substrate) are then welded or crimped together to complete the circuit as required. This approach requires the substrate to be custom designed and made, with appropriate holes and contacts, and so is expensive and hence mainly only used for higher cost applications.
In a more recent approach, known as ‘flip chip’, the component is placed on the substrate in inverted condition, i.e. with the contacts on the lower surface of the component, adjacent the substrate. Contacts on the component or on the substrate are raised or ‘bumped’ to provide a protruding electrical connection between the component and substrate circuit. Several different procedures are known for forming the bumps, including palladium (Pd) bumping, nickel gold (NiAu) bumping, polymer bumping, solder bumping, possibly with under bump metallisation (UBM).
In a further approach, components are accurately pre-mounted on a supporting sheet or tape, known as an interposer, that has appropriately located contacts for electrical connection to the components. The interposer with mounted components is then placed on the substrate and electrically connected thereto via contact pads on the interposer and substrate. The contact pads are relatively large, typically several square millimetres in area, so placement of the interposer on the substrate need not be carried out with great accuracy. The contacts are usually pressed or crimped together to complete the electrical connection between the interposer, and hence components, and the substrate. This technique has the drawbacks of using an additional component, namely the interposer, and of requiring an additional processing step.